Magnetic encoders are used for detecting the rotational position of the rotating shaft of a servo motor or the like. Such a magnetic encoder is disclosed in, e.g., Patent Documents 1 through 3. In the position detection device disclosed in Patent Document 1, a pair of Hall elements is arranged at an angular interval of 90° so that sinusoidal signals having a phase difference of 90° are detected in accompaniment with the rotation of a bipolarly magnetized magnet ring. The error component due to turbulence or the like is reduced in half by using the difference in the outputs of the Hall elements.
In the sine-cosine output sensor disclosed in Patent Document 2, four Hall elements are arranged at equiangular intervals of 90° along the external periphery of a bipolarly magnetized magnet ring, and the detection accuracy of the rotational position is improved based on the output of the Hall elements.
In the encoder device disclosed in Patent Document 3, a 12-bit absolute value output having a resolution of 4096 (64×64) is obtained using a two-pole magnetic encoder and a 64-pole magnetic encoder in order to detect the rotational position of the rotating shaft with good precision. In this magnetic encoder, 6 upper bits are generated by the two-pole magnetic encoder, and 6 lower bits are generated by the 64-pole magnetic encoder.
[Patent Document 1] Japanese Laid-open Patent Application No. S58-162813
[Patent Document 2] Japanese Laid-open Patent Application No. 2001-050774
[Patent Document 3] Japanese Laid-open Utility Model Application No. H06-10813
Here, in a magnetic encoder in which a magnetic ring is used, fluctuations in the magnetic flux density caused by rotational runout of the magnetic ring cause the precision of the detection angle to be reduced. Also, the nonuniformity of magnetic flux density of each magnetic pole causes the precision of the detection angle to be reduced in the case that a multi-pole magnetic ring is used.
The following causes also result in a reduction in the precision of the detection angle when a bipolarly magnetized magnetic ring and multi-polarized magnetic ring are used in the manner of the encoder device disclosed in Patent Document 3. In other words, it is possible that the Hall element or another magnetic detecting element for detecting the magnetic flux of a multi-polarized magnetic ring will be affected by magnetic flux from adjacently arranged bipolarly magnetized magnetic rings, the error component of the detection signal will be multiplied, and precision of the detection angle will be reduced.
In the encoder device disclosed in Patent Document 3, the precision of the two-pole magnetic encoder must be equivalent to the 6 bits of the 64-pole magnetic encoder. The precision of the two-pole magnetic encoder must therefore be further increased in order to obtain output having higher precision, and increasing precision is therefore difficult. The start points of the output signal of the two-pole magnetic encoder and the output signal of the 64-pole magnetic encoder must be aligned, and problems are presented in that time is required to make such adjustments.